Reactor structural member and method of suppressing corrosion of the same

ABSTRACT

A photocatalytic substance having the properties of an n-type semiconductor is deposited on a surface of a metal base made of a stainless steel or Inconel. When necessary, the hydrogen concentration of the reactor water is increased. A current produced by the photocatalytic substance when the same is irradiated with light or radioactive rays in a nuclear reactor flows through the metal base to reduce corrosion current. When necessary, the photocatalytic substance is provided on its surface with at least one of Pt, Rh, Ru and Pd.

The present application is a divisional of U.S. application Ser. No.09/599,027, filed Jun. 22, 2000, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to reactor structural members, namely,materials used for constructing a reactor, resistant to corrosion in areactor primary system of a nuclear power plant, and a method ofsuppressing corrosion of reactor structural members.

2. Description of the Related Art

In a BWR power plant, reactor water contains oxygen and hydrogenperoxide produced by the radiolysis of water in a radiation field. It isknown that oxygen and hydrogen peroxide contained in the reactor watercause intergranular stress-corrosion cracking (IGSCC) in the structuralmembers of stainless steels and nickel-base alloys in an environment ofthe high-temperature, high-pressure water of nuclear reactors.Initiation of stress-corrosion cracks and propagation of cracks aredependent on corrosion potential. Reduction of oxygen and hydrogenperoxide reduces the corrosion potential of a member. The lower thecorrosion potential, the lower is the possibility of crack initiationand propagation of cracks.

A hydrogen injection method that injects hydrogen through a feedwatersystem into a nuclear reactor to reduce oxygen and hydrogen peroxidecontained in reactor water is a method that has been practically appliedto prevent the stress-corrosion cracking in some domestic and foreignnuclear power plants. However, the hydrogen injection method is attendedwith an adverse effect caused by the reaction of injected hydrogen withN-16 produced by nuclear reaction to produce volatile ammonia. Thevolatile ammonia is liable to enter the steam system, which increasesthe dose rate of the turbine system. When hydrogen is injected throughthe feedwater system into the reactor water, oxygen must be injected toreduce excess hydrogen in the off-gas. system by recombination andvarious facilities are necessary therefor.

A corrosion potential reducing method recently proposed to reduce thecorrosion potential of structural members without causing those problemsadds a noble metal, such as platinum, to the reactor water to depositthe noble metal on the surfaces of the structural members and reducesthe corrosion potential by injecting a small amount of hydrogen into thereactor water. This corrosion potential reducing method utilizes theproperty of the noble metal, such as platinum, to selectively arrest thereversible. reaction of hydrogen having a low potential with anintention to reduce the corrosion potential by injecting a small amountof hydrogen into the reactor water.

However, when this method is practiced in a nuclear power plant, thenoble metal adheres also to a zirconium oxide film contained in thefuel, which promotes the oxidation and hydrogenation of the fuelmaterial. Moreover, the interaction of hydrogen injected into thereactor water and N-16 produced by nuclear reaction is promoted,volatile ammonia enters the steam system and the dose rate of theturbine system increases.

Furthermore, since a noble metal chemical containing impurities is usedin a high concentration, the quality of the reactor water isdeteriorated adversely affecting the soundness of the fuel materials.Thus, the noble metal injection method now in use exerts adverse effectson the maintenance of water quality, the reduction of radioactivitytransition and the enhancement of the degree of burn-up of the fuel. Toreduce such adverse effects, it is desired to develop a method thatinjects a small amount of the noble metal and a method that uses asubstance other than the noble metal.

On the other hand, ions contained in feedwater adheres as looseparticles to the surfaces of members disposed within the nuclear reactorwhen the feedwater has a high iron concentration. If the noble metaladhering to the structural members adheres to those particles, the noblemetal adheres to the fuel when the particles separates from the surfacesof the structural members and promotes the oxidation and hydrogenationof the fuel materials.

As mentioned above, when hydrogen is injected into the reactor water bythe known stress-corrosion cracking preventing method, the hydrogen andN-16 produced by nuclear reaction interact to produce volatile ammonia.The volatile ammonia thus produced is liable to enter the steam systemto increase the dose rate of the turbine system. Various facilities arenecessary to reduce excess hydrogen in the off-gas system byrecombination.

When a noble metal is deposited on the surfaces of structural members bythe method that adds the noble metal to the reactor water to reduce thecorrosion potential by injecting a small amount of hydrogen into thereactor water, the noble metal adheres also to the zirconium oxide filmsand the oxidation and hydrogenation of the fuel materials are promoted.Further more, since a noble metal chemical containing impurities is usedin a high concentration, the quality of the reactor water isdeteriorated adversely affecting the soundness of the fuel materials.

Ion contained in the feedwater adheres in loose particles to thesurfaces of members disposed within the nuclear reactor when thefeedwater has a high iron concentration. If the noble metal adhering tothe structural members adheres to those particles, the noble metaladheres to the fuel when the particles separate from the surfaces of thestructural members.

SUMMARY OF THE INVENTION

The present invention has been made to solve those problems in therelated art and it is an object of the present invention to provide areactor structural member and a method of reducing corrosion of thereactor structural member capable of reducing the respective amounts ofhydrogen and noble metal to be injected into the reactor water toprevent stress-corrosion cracking, of reducing the transfer ofradioactivity to the turbine system, of reducing excess hydrogen in theoff-gas system, of reducing the amount of a noble metal adhering to thesurface of the fuel material to the least necessary extent, and ofreducing the corrosion potential of members of the primary system of anuclear reactor by suppressing the production of loose iron particles onthe surface of the fuel without promoting the oxidation andhydrogenation of the fuel material.

According to the present invention, a reactor structural member has asurface provided with a corrosion potential reducing substance, such asa photocatalytic substance that produces electromotive force whenexposed to light or radioactive rays in a nuclear reactor, a metal or ametal compound that forms such a photocatalytic substance under acondition specified by a temperature and a pressure in the nuclearreactor. Preferably, the corrosion potential reducing substance isformed as a particle having a surface provided with at least one of Pt,Rh, Ru and Pd.

The light in the nuclear reactor is a radiation including ultravioletrays as a principal component, which is known as Cherenkov rays producedby the nuclear fission of a fuel material in a water-cooled nuclearreactor. The radioactive rays in a nuclear reactor are electromagneticwaves and corpuscular beams produced by the nuclear fission of the fuelmaterial, such as α rays, β rays, γ rays and neutron beams.

The photocatalytic substance is a substance having a property of ann-type semiconductor, such as any one of compounds including TiO₂, ZrO₂,PbO, BaTiO₃, Bi₂O₃, ZnO, WO₃, SrTiO₃, Fe₂O₃, FeTiO₃, KTaO₃, MnTiO₃, andSnO₂. These compounds are stable in a high-temperature, high-pressure,radioactive environment, and do not significantly increase the migrationof radioactivity to the turbine system, and do not significantly promotethe oxidation and hydrogenation of the fuel material. The corrosionpotential of the structural members of the primary system of a nuclearreactor can be controlled by attaching any one of those compounds to thestructural members of the nuclear reactor or by forming a film of thecompound on the surface of the structural members of the nuclearreactor. Preferably water quality is controlled. The radioactivity ofthe fuel material and the activated compound is low.

Among those compounds, TiO₂ and ZrO₂ are particularly preferablephotocatalytic substances.

Although the compounds may be used in the form of oxides as mentionedabove which are photocatalytic substances as they are, phtocatalystforming substances that produce the foregoing compounds under ahigh-temperature, high-pressure condition in the nuclear reactor, moreconcretely, in an environment of 285° C. and 70 atm, such as metals andmetal hydrates, may be used. More concretely, possible metals and metalhydrates are, for example, metal Ti, metal Zr, Ti hydrate and Zrhydrate.

Those photocatalytic substances or photocatalyst forming substances areused instead of the noble metal, such as Pt. Those photocatalyticsubstances and photocatalyst forming substances may be used as particleshaving surfaces on which at least one of Pt, Rh, Ru and Pd is partiallyattached.

The photocatalytic substance, the photocatalyst forming substance, thephotocatalytic substance combined with a noble metal, such as Pt, or thephotocatalyst forming substance combined with a noble metal, such as Pt,is attached to the surface of a structural member of a nuclear reactor.A film of the substance may be formed on the surface of a structuralmember of a nuclear reactor.

A method of securely attaching the corrosion potential reducingsubstance, i.e., the photocatalytic substance, the photocatalyst formingsubstance, the photocatalytic substance combined with a noble metal orthe photocatalyst forming substance combined with a noble metal, to thesurface of an objective structural member to be protected from corrosionsupplies the corrosion potential reducing substance into the coolingwater while the nuclear reactor is in rated operation, in a start-upstage, in a shut-down stage, while a plant loaded with the fuel isstopped without providing any thermal output or while the plant is notloaded with any fuel, and circulates the cooling water to coat thesurface of the structural member of the nuclear reactor with thecorrosion potential reducing substance.

The corrosion potential reducing substance may be sprayed over thesurface of the objective structural member to form a film of thecorrosion potential reducing substance in a thickness in the range of0.1 to 1 μm by using a robot after removing the fuel from the nuclearreactor and decontaminating the nuclear reactor for periodic inspection.The film of the corrosion potential reducing substance having apredetermined thickness can be formed on the surface of the structuralmember by a method that sprays a liquid containing the corrosionpotential reducing substance over the surface of the structural memberin a film and drying the film, a thermal spraying method, a physicalvapor deposition (PVD) method or a chemical vapor deposition (CVD)method.

When the corrosion potential reducing substance is applied to thesurface of a structural member of a nuclear reactor after providing thesame with a hydrophilic property or when a mixture prepared by mixingthe corrosion potential reducing substance and a binder is applied tothe surface of a structural member of a nuclear reactor, the initialadhesion of the corrosion potential reducing substance to the surface ofthe structural member can be enhanced.

Desirably, the amount of the corrosion potential reducing substanceapplied to the structural member or the thickness of a film of thecorrosion potential reducing substance formed on the surface of thestructural member is designed to make the photocatalytic substanceproduce a current of a current density not lower than the sum of thelimiting current densities of oxygen and hydrogen peroxide contained inthe reactor water.

When a corrosion oxide film formed on the surface of the structuralmember is a single layer of a p-type semiconductor, the corrosionpotential reducing substance may be applied to the corrosion oxide film,or a film of the corrosion potential reducing substance may be formed onthe corrosion oxide film. When a corrosion oxide film consists of anouter layer having the property of an n-type semiconductor other than aphotocatalytic substance and an inner layer having the property of ap-type semiconductor, it is desirable to apply the corrosion potentialreducing substance to the corrosion oxide film or to form a film of thecorrosion potential reducing substance on the corrosion oxide film aftermaking the outer layer of the n-type semiconductor unstable or removingthe outer layer of the n-type semiconductor.

The outer layer of the n-type semiconductor other than thephotocatalytic substance can be made unstable by a method that increasesthe hydrogen concentration of the reactor water. The outer layer of then-type semiconductor other than the photocatalytic substance can beremoved by a chemical decontaminating method, an electrolyticdecontaminating method or a laser decontaminating method. When the outerlayer of the n-type semiconductor is decontaminated by submerged laserirradiation, a compressive stress effective in preventing IGSCC can beinduced in the structural member of the nuclear reactor by thedecontaminating and peening action of a laser beam.

According to the present invention, it is desirable to control the ironconcentration of the feedwater by placing a condensate purifier in thecondensing system of the nuclear reactor to suppress the loosedeposition of hematite on the surface of the fuel material. A suitablecondensate purifier includes a filter device and a demineralizer device.

According to the present invention, to apply a noble metal effectivelyto the surface of a member of the primary system of a nuclear reactorand to limit the amount of the noble metal adhering to the surface ofthe fuel material to the least necessary extent, the iron concentrationof the feedwater is adjusted to about 0.1 ppb or below to inhibit theformation of loose hematite on the fuel. When the iron concentration ofthe feedwater is about 0.1 ppb or below, nickel ion concentration isgreater than 0.2 ppb. Under this condition, the amount of the noblemetal to be injected into the nuclear reactor may be 1/10 of that of thenoble metal used by the related art for the same purpose. The ironconcentration of the feed water can be reduced to about 0.1 ppb or belowby using the condensate purifier including the filter device and thedemineralizer device.

It is desirable, while the iron concentration of the feed water is thuscontrolled, that one or some of Pt, Rh, Ru or Pd are made to adhere tothe surface of the structural member of a nuclear reactor having acorrosion oxide film in a weight per unit area of 0.1 μg/cm² or aboveand water quality is controlled so that the oxygen/hydrogen molar ratioof the reactor water is in the range of 0.4 to 0.5.

The potential of the member can be reduced by a method that increasesthe hydrogen concentration of water. The hydrogen concentration of watercan be increased by injecting hydrogen into the nuclear reactor or byinjecting methanol that produces hydrogen in the nuclear reactor. Tostabilize the catalyst and to maintain the effect of the catalyst, it isdesirable to limit the iron concentration of the feedwater to 0.1 ppb orbelow.

The iron concentration of the feedwater can be reduced to 1 ppb or belowby using the condensate purifier including the filter device and thedemineralizer device. The performance of the filter device is important.A hollow fiber filter satisfies filtering requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description taken inconnection with the accompanying drawings, in which:

FIG. 1 is a graph showing the variation with time of the corrosionpotential of a structural member of a stainless steel (SUS304, JIS)coated with a TiO₂ film when exposed to ultraviolet rays;

FIG. 2 is a typical view of assistance in explaining a reaction thatoccurs when a metal base coated with an n-type semiconductor film isexposed to light rays, such as ultraviolet rays;

FIG. 3 is a graph showing the variation with time of the corrosionpotential of a structural member of a stainless steel (SUS304, JIS)coated with a TiO₂ film when exposed to ultraviolet rays for the amountof hydrogen injected into a nuclear reactor;

FIG. 4 is a diagrammatic view of a BWR nuclear plant;

FIG. 5 is a typical view of assistance in explaining a method of forminga film on the surface of a metal base by a plasma spraying apparatus;

FIG. 6 is a graph showing the variation of the corrosion potential of astructural member of a stainless steel (SUS304, JIS) coated with asemiconductor film when the structural member is exposed to ultravioletrays;

FIG. 7 is a graph showing the variation with time of the corrosionpotential of a structural member of a stainless steel (SUS304, JIS)coated with an oxide film when exposed to ultraviolet rays;

FIG. 8 is a graph showing the dependence of the corrosion potential of astructural member of a stainless steel (SUS304, JIS) coated with a TiO₂film on the thickness of the TiO₂ film;

FIG. 9 is graph of assistance in explaining the effect of a small amountof Pt made to adhere to a TiO₂ film formed on a surface of a structuralmember of a stainless steel (SUS304, JIS) on reducing corrosionpotential when the structural member is exposed to ultraviolet rays;

FIG. 10 is a graph of assistance in explaining the effect of SiO₂ usedas a binder in forming a TiO₂ film on a structural member on corrosionpotential;

FIG. 11 is a typical view of assistance in explaining the migration ofsurplus electrons from an n-type semiconductor to a metal base caused byCherenkov rays;

FIG. 12 is a graph showing the variation with time of the corrosionpotential of a member of a stainless steel (SUS304, JIS) coated with aCr₂O₃ film of a p-type semiconductor film, and having a TiO₂ film formedon the Cr₂O₃ film and that of a member of a stainless steel (SUS304,JIS) coated with an Fe₂O₃ film of an n-type semiconductor film, andhaving a TiO₂ film formed on the Fe₂O film when irradiated withultraviolet rays;

FIG. 13 is a typical view of assistance in explaining the difference inreaction between a member of a stainless steel (SUS304, JIS) coated witha Cr₂O₃ film of a p-type semiconductor film, and having a TiO₂ filmformed on the Cr₂O₃ film and that of a member of a stainless steel(SUS304, JIS) coated with an Fe₂O₃ film of an n-type semiconductor filmand having a TiO₂ film formed on the Fe₂O₃ film when irradiated withultraviolet rays;

FIGS. 14A and 14B are typical views of assistance in explaining a methodof forming a photocatalyst film on a member having a metal base, ap-type semiconductor film formed on the metal base, and an n-typesemiconductor film, which is not a photocatalyst film, formed on thep-type semiconductor film after removing the n-type semiconductor film;

FIG. 15 is a typical view of assistance in explaining a reason that itis desirable, when a member has a metal base, a p-type semiconductorfilm formed on the metal base, and an n-type semiconductor film, whichis not a photocatalyst film, formed on the p-type semiconductor film, toremove the n-type semiconductor film, which is not a photocatalyticfilm, and to form an n-type semiconductor film, which is a photocatalystfilm, on the p-type semiconductor film;

FIG. 16 is a typical view of assistance in explaining, when a member hasa metal base and an n-type semiconductor film, which is not aphotocatalyst film, a process of forming a p-type semiconductor film onthe metal base by a chemical reaction after the n-type semiconductorfilm has been removed by chemical decontamination or electrolyticdecontamination;

FIG. 17 is a graph showing the relation between the amount of a noblemetal attached to the surface of a structural member of a nuclearreactor, corrosion potential and the iron concentration of reactorwater;

FIG. 18 is a diagrammatic view of a BWR nuclear plant with a feedwaterpurifier;

FIG. 19 is diagrammatic view of a BWR nuclear plant with ahydrogen/alcohol injecting system; and

FIG. 20 is a diagrammatic view of a BWR nuclear plant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, the iron concentration of feed water in BWR power plants inJapan is several hundreds parts per trillion (ppt) or above and in BWRpower plants in America is 1000 ppt or above. Therefore, the amount of anoble metal per unit area necessary to stably control the potential ofthe structural members of the primary system of a nuclear reactor isseveral micrograms per square centimeters or above and hence one BWRneeds several kilograms of a noble metal.

When any particles of corrosion products do not deposit from the reactorwater on the surfaces of the structural members of the primary system,the necessary amount of a noble metal per unit area is on the order of0.1 μg/cm² and hence one BWR needs 100 g or below of a noble metal.Therefore, the amount of the noble metal that adheres to the fuel can bereduced to 1/10 of the amount of the noble metal necessary. whenparticles of corrosion products deposit on the surfaces of thestructural members of the primary system and the oxidation andhydrogenation of the fuel material can be prevented. Since theconcentration of the noble metal chemical can be reduced to 1/10 of theconcentration of the same necessary when particles of corrosion productsdeposit on the surfaces of the structural members of the primary system,the deterioration of water quality and the transfer of N-16 to theturbine system can be suppressed.

The reduction of the iron concentration of the feedwater brings abouteffects of stabilizing the adhesion of the noble metal to the surface ofthe structural member and maintaining the effect of the noble metal.Such effects can be exercised similarly when a photocatalyst is used.

The effect of the photocatalytic substance employed in the presentinvention, such as TiO₂ or ZrO₂, is the same as that of the noble metalin stopping hydrogen injection and reducing the amount of hydrogen to beinjected into the nuclear reactor.

It is feared that a noble metal, such as Pt, attached to a part of thesurface of the photocatalytic substance to enhance the effect of thephotocatalytic substance, such as TiO₂, adheres to the surface of thefuel material and affects the oxidation and hydrogenation of the fuelmaterial. However, the amount of the noble metal attached to the surfaceof the photocatalytic substance is 1/10 or below of the amount of thenoble metal necessary when the noble metal is attached directly to theZrO₂ film of the fuel material. Therefore, the influence of the noblemetal on the fuel material is practically negligible.

Weld Ni-base alloy, for which corrosion is a significant problem, liesin the bottom section of a reactor vessel of a BWR plant. The depth ofwater to the bottom section is 5.3 m at the maximum. Since about 10% ofultraviolet rays included in Cherenkov rays reaches the bottom section,a sufficient amount of photons necessary for exciting the photocatalystreaches the reactor structural members facing the reactor core.

A TiO₂ photocatalyst, i.e., an n-type semiconductor, has electrochemicalproperties similar to those of ZrO₂. It is known that the TiO₂ catalystexercises its photocatalytic effect to promote anodic reaction and itscorrosion potential drops. It is known that the corrosion potential of aCr₂O₃ film and a NiO film, i.e., corrosion oxide films, formed on thesurface of the weld Ni-base alloy rises when the same are p-typesemiconductors.

To lower the potential of the member for IGSCC corrosion suppression, aTiO₂ photocatalyst, which is an n-type semiconductor, is applied to thecorrosion oxide film formed on the surface of the molten Ni-base alloyto shield the corrosion oxide film, which is a p-type semiconductor,from Cherenkov rays to suppress the rise of potential, and the potentialof the Ni-base alloy can be lowered by the effect of reduction of thecorrosion potential of the TiO₂ photocatalyst.

The TiO₂ photocatalyst has a high photocatalytic effect on an Fe-basealloy on which an Fe₂O₃ film, which is an n-type semiconductor film, isformed as an outer layer, such as a stainless steel. Therefore, thecorrosion potential can be lowered by selectively using a suitablephotocatalytic substance.

When an Fe-base alloy on which an inner layer of a p-type semiconductorand an outer layer of an n-type semiconductor are formed, such as astainless steel, is used, the n-type semiconductor of an oxide formingthe outer layer is made unstable to expose the p-type semiconductor ofan oxide in a contact interface by increasing the hydrogen concentrationof the reactor water, or the n-type semiconductor of an oxide formingthe outer layer can be removed by decontamination. The corrosionpotential of the member can be further lowered by bringing the p-typesemiconductor of an oxide, and an n-type semiconductor, such as TiO₂,i.e., a powerful photocatalytic substance, into contact.

When the intensity of a laser beam in water is increased to remove theoxide film, a compressive stress effective in preventing IGSCC can beinduced in the structural member of the nuclear reactor by the peeningeffect of the laser beam.

When it is necessary to suppress IGSCC by lowering the corrosionpotential of Fe-base and Ni-base alloy members by using a photocatalyticsubstance, such as TiO₂, the effect of the photocatalytic substance isfurther enhanced by increasing the hydrogen concentration of the reactorwater, because dissolved oxygen is more active in receiving electronsthan hydrogen ions. Therefore, it is effective to reduce dissolvedoxygen by increasing the hydrogen concentration of the reactor water. Itis desirable to inject a molar amount of oxygen corresponding to ½ ofthe molar amount of hydrogen to be injected into a system after thesteam system to make oxygen and excess hydrogen interact.

Under conditions simulating water quality control in a practical plant,it was found that the corrosion potential could be lowered to −230 mV vsSHE effective in suppressing IGSCC by using feedwater having a hydrogenconcentration in the range of 0.2 to 0.4 ppm when TiO₂ combined with Ptand TiO₂ as a phtocatalyst are applied to a part of the surface of aweld Ni-base alloy in a 1 μm thick film. When the hydrogen concentrationis in that range the migration of radioactivity to the turbine systemdoes not increase.

When applying the photocatalytic substance to a member for whichcorrosion must be suppressed, it is important to avoid the adhesion ofthe photocatalytic substance to the fuel material, to control thethickness of the film of the photocatalytic substance, and to suppressthermal influence on the member to the least extent.

A photocatalytic substance or a photocatalyst forming substance can beapplied to a member for which corrosion must be suppressed bycirculating cooling water while the plant loaded with the fuel isstopped without providing any thermal output or while the fuel is takenout and immersed in cooling water. The photocatalytic substance or thephotocatalyst forming substance may be sprayed over the surface of anobjective structural member by using a robot after removing the fuelfrom the nuclear reactor and decontaminating the nuclear reactor forperiodic inspection.

A film of the photocatalytic substance or the photocatalyst formingsubstance having a predetermined thickness can be formed on the surfaceof the structural member by a method that sprays a liquid containing thesame substance over the surface of the structural member in a film anddrying the film, a thermal spraying method, a PVD method or a CVDmethod.

When a photocatalytic substance, such as TiO₂, is applied to the surfaceof a structural member after providing the same with a hydrophilicproperty or when a mixture prepared by mixing the photocatalyticsubstance and a binder, such as SiO₂ is applied to the surface of astructural member, the initial adhesion of the photocatalytic substanceto the surface of the structural member can be enhanced.

A TiO₂ film of a thickness in the range of a fraction of 0.1 μm to 1 μmis capable of completely absorbing Cherenkov rays and has a sufficientmechanical strength.

FIG. 1 is a graph showing the variation with time of the corrosionpotential of a heat-affected part of a structural member of a stainlesssteel (SUS304, JIS) coated with a TiO₂ film by thermal spraying when theheat-affected part is exposed to ultraviolet rays. In FIG. 1, both thevariation of the corrosion potential of the heat-affected zone coatedwith the TiO₂ film and that of a part not coated with any film areshown. As obvious from FIG. 1, the corrosion potential of theheat-affected zone coated with the TiO₂ film decreases when theheat-affected zone is irradiated with ultraviolet rays. The corrosionpotential of the zone not coated with any TiO₂ film does not decreasewhen the same part is irradiated with ultraviolet rays. Thus, it isknown that the TiO₂ film formed by thermal spraying cause the corrosionpotential to decrease.

FIG. 2 is a typical view of assistance in explaining a reaction thatoccurs when a metal base 2 of a stainless steel or Inanely coated withan n-type semiconductor film 1 is exposed to light rays, such asultraviolet rays. As shown in FIG. 2, the n-type semiconductor film 1has locally unbonded surplus electrons 3. When the n-type semiconductorfilm 1 is irradiated with effective light rays 4, such as ultravioletrays, the n-type semiconductor forming the n-type semiconductor film 1is excited. Consequently, electrons are allowed to move easily, migratefrom the n-type semiconductor film 1 to the metal base 2 and are thusdissipated. This reaction makes anode current flow easily, so that thecorrosion potential decreases.

FIG. 3 is a graph showing the variation with time of the corrosionpotential of a structural member of a stainless steel (SUS304, JIS)coated with a TiO₂ film when exposed to ultraviolet rays for an amountof hydrogen injected into reactor water. The corrosion potential of thestructural member of SUS304 decreases when the structural member isirradiated with ultraviolet rays under a condition where hydrogen is notinjected into the reactor water. However, when the structural member isirradiated with ultraviolet rays under a condition where hydrogen isinjected into the reactor water, the corrosion potential of thestructural member of SUS304 decreases more effectively.

FIG. 4 is a diagrammatic view of a BWR nuclear plant including apressure vessel 5, a feedwater line 6, a purifying system 7 and arecirculation line 8. A recirculation system injection line 9 isconnected to the recirculation line 8 and a feedwater system injectionline 10 is connected to the feedwater line 6. A corrosion potentialmeasuring apparatus 11 is installed. A semiconductor composition can beinjected into the reactor water.

The reactor water is supplied through the recirculating line 8 to thecorrosion potential measuring apparatus 11. The corrosion potentialmeasuring apparatus 11 measures corrosion potential in thehigh-temperature reactor water. When applying a p-type semiconductor toa reactor structural member, the injection lines 9 and 10 for injectinga semiconductor composition into the reactor water are actuated toinject the semiconductor composition into the reactor water. Thesemiconductor composition may be injected into the reactor water whilethe nuclear reactor is in normal operation, while the nuclear reactor isstopped or while the nuclear reactor is in a start-up stage, providedthat an objective part of the reactor structural member to be subjectedto corrosion potential reduction is exposed to the reactor water. Therelation between injection time, temperature and concentration may bedetermined beforehand through experiments and the relation may be usedfor controlling a semiconductor composition injecting operation or thesemiconductor composition injecting operation may be controlled bymonitoring the process of the semiconductor composition injectingoperation by the corrosion potential measuring apparatus and thesemiconductor composition may be injected into the reactor waterconfirming the reduction of corrosion potential.

Although the semiconductor composition is injected into the reactorwater through the recirculation system in this embodiment, thesemiconductor composition may be injected into the reactor water throughany part connected to the nuclear reactor, such as a feedwater system, aresidual heat removing system, a nuclear reactor cleaning system, asampling line or the like. The semiconductor composition may beintroduced into the nuclear reactor by a method that holds a sinteredsemiconductor composition compact that discharges a semiconductorcomposition when irradiated with ultraviolet rays in the reactor water.

FIG. 5 is a typical view of assistance in explaining a method of forminga film on the surface of a metal base by a plasma spraying apparatus. Adc arc 14 is produced between a cathode 12 and an anode nozzle 13. Anorifice gas 15 supplied from behind the cathode 12 is heated andexpanded by the dc arc 14, and a plasma jet 16 is jetted through theanode nozzle 13. Powder 17 of a thermal spray material is carried by agas into the plasma jet 16. The powder 17 of the thermal spray materialis heated, is accelerated by the plasma jet 16 and impinges on a surfaceof a metal base 2 to form a photocatalyst film 18 on the surface of themetal base 2.

FIG. 6 is a graph showing the variation of the corrosion potential of astructural member of a stainless steel (SUS304, JIS) coated with asemiconductor film when the structural member is exposed to ultravioletrays. An anode current produced by the photoelectrochemical reaction ofa semiconductor must be higher than a cathode limiting current density,i.e., the density of a cathode current produced by the oxidizingcomponents, such as oxygen and hydrogen peroxide, of the reactor waterto reduce the corrosion potential. The anode current produced by thephotoelectrochemical reaction of the semiconductor is dependent on theintensity of light and the mass of deposit per unit area of thesemiconductor composition. The cathode current is dependent on theconcentration of the oxidizing materials, such as oxygen and hydrogenperoxide, contained in the reactor water. For example, it is estimatedthat oxygen concentration and hydrogen peroxide concentration in abottom part of the nuclear reactor are 200 ppb. A limiting currentdensity of the cathode current resulting from the oxidizing substancescalculated taking into account the amount of the oxidizing substancesand flow conditions in the bottom part of the nuclear reactor is about18 A/m². To reduce corrosion potential, the anode current must be higherthan about 18 A/m². When a current of 18 A/m² or higher is produced bythe photoelectrochemical reaction, corrosion potential decreases.

FIG. 7 is a graph showing the variation with time of the corrosionpotential of a structural member of a stainless steel (SUS304, JIS)coated with an oxide film containing a semiconductor composition whenexposed to ultraviolet rays. The oxide film takes in the semiconductorcomposition as it grows on the surface of the structural member.

This method, instead of directly applying a semiconductor composition toa structural member of a nuclear reactor, dissolves or suspends asemiconductor composition in the reactor water and leaves the reactorwater as it is for a fixed time to make the semiconductor compositionadhere to the surface of the structural member.

Test pieces for the measurement of data shown in FIG. 7 were prepared byimmersing pieces of a stainless steel (SUS304, JIS) in ahigh-temperature titanium oxide solution to form a TiO₂ film on thesurfaces of the pieces. As obvious from FIG. 7, the corrosion potentialof the test pieces decreases when the same are irradiated withultraviolet rays, and the reduction of the corrosion potential increaseswith immersing time.

FIG. 8 is a graph showing the dependence of the corrosion potential of astructural member of a stainless steel (SUS304, JIS) coated with a TiO₂film on the thickness of the TiO₂ film. The corrosion potential startsdecreasing when the thickness of the TiO₂ film increases to 0.1 μm, andthe curve indicating the variation of the corrosion potential with thethickness of the TiO₂ film levels off after the thickness increasesbeyond 1 μm. Therefore, the effective thickness of the TiO₂ film is inthe range of 0.1 to 1 μm. It goes without saying that the TiO₂ film of athickness not smaller than 1 μm reduces the corrosion resistanceeffectively.

FIG. 9 is graph of assistance in explaining the effect of Pt made toadhere in a mass per unit area of 0.1 μg/cm² to a TiO₂ film formed on asurface of a structural member of a stainless steel (SUS304, JIS) onreducing corrosion potential when the structural member is exposed toultraviolet rays. As obvious from FIG. 9, Pt added to the TiO₂ film iseffective in increasing the reduction of the corrosion potential. Datashown in FIG. 9 proves that the addition of Pt to the TiO₂ film enhancesthe corrosion potential reducing efficiency of the TiO₂ film.

FIG. 10 is a graph of assistance in explaining the effect of SiO₂ usedas a binder in forming a TiO₂ film on a structural member on corrosionpotential. When any binder is not used, the mass per unit area of theTiO₂ film decreases with time and, consequently, the corrosion potentialincreases with time. When a binder is used, the adhesion of the TiO₂film to the surface of the structural member increases and the corrosionpotential is held on a low level. It is known from FIG. 10 that theaddition of a binder to the catalyst is effective in maintainingcorrosion potential reducing ability.

FIG. 11 is a typical view showing the migration of surplus electrons 3from an n-type semiconductor 1 to a metal base 2 caused by Cherenkovrays. Many charged particles 19 are flying about in a nuclear reactor.It is generally known that light rays 4 called Cherenkov radiation areproduced when the charged particles are decelerated. Cherenkov radiationhas wavelengths in a wide wavelength range owing to difference in energybetween the decelerated charged particles. Cherenkov radiation includesrays of wavelengths in the ultraviolet region. Therefore, it isconsidered that Cherenkov radiation is able to excite photocatalysts.

FIG. 12 shows the variation with time of the corrosion potential of amember of a stainless steel (SUS304, JIS) coated with a Cr₂O₃ film of ap-type semiconductor film, and having a TiO₂ film formed on the Cr₂O₃film and that of a member of a stainless steel (SUS304, JIS) coated witha Fe₂O₃ film of an n-type semiconductor, and having a TiO₂ film formedon the Fe₂O₃ film when irradiated with ultraviolet rays. The respectivecorrosion potentials of both the members are reduced when irradiatedwith ultraviolet rays, which verifies the effect of the TiO₂ film on thereduction of corrosion potential. The corrosion potential reducingeffect of the combination of the TiO₂ film and the p-type semiconductorfilm is greater than that of the combination of the TiO₂ film and then-type semiconductor film. FIG. 13 is a typical view of assistance inexplaining the principle of a phenomenon illustrated by FIG. 12. Whereasan n-type semiconductor film 1 locally has surplus electrons 3, a p-typesemiconductor film 21 locally has unfilled bonds 20. The surpluselectrons 3 of the n-type semiconductor film 1 excited by light rays 4migrate easily into the unfilled bonds 20 of the p-type semiconductorfilm 21 contiguous with the n-type semiconductor film 1. The easiness ofthe migration of the electrons 3 into the p-type semiconductor film 21is higher than that of electrons into the metal base 2 in whichelectrons are arranged regularly. Thus, the easiness of flow of anodecurrent is promoted by forming a p-type semiconductor film on an n-typesemiconductor film, and the reduction of corrosion potential by theformation of the TiO₂ film on the p-type semiconductor film is greaterthan that of the same by the formation of the TiO₂ film directly on themetal base 2.

FIGS. 14A and 14B are typical views of assistance in explaining a methodof forming an n-type semiconductor film 1, i.e., a photocatalyst film,on a member having a metal base 2, a p-type semiconductor film 21 formedon the metal base 2, and an n-type semiconductor film 22, which is not aphotocatalyst film, formed on the p-type semiconductor film afterremoving the n-type semiconductor film 22. This arrangement of thep-type semiconductor film and the n-type semiconductor film 1, i.e., acatalytic film, enhances the corrosion potential reducing effect. Onlythe n-type semiconductor film 22, which is not a photocatalyst film, canbe removed by a method that makes the n-type semiconductor film 22unstable by injecting a large quantity of hydrogen into the reactorwater 23 to reduce dissolved oxygen. Only the n-type semiconductor film22, which is not a photocatalyst film, can be removed by adecontamination process. The corrosion potential reducing effect of then-type semiconductor film 1, i.e., the photocatalyst film, is enhancedwhen the same is formed on the p-type semiconductor film 21 afterremoving the n-type semiconductor film 22.

FIG. 15 is a typical view of assistance in explaining a reason that itis desirable to remove the n-type semiconductor film 22, which is not aphotocatalyst film, formed on the p-type semiconductor film 21 shown inFIGS. 14A and 14B. When the n-type semiconductor film 1, which is aphotocatalyst film, on the n-type semiconductor film 22, which is not aphotocatalyst film, surplus electrons 3 excited by light rays 4 must bedissipated in the n-type semiconductor film 22 having many surpluselectrons 3. The easiness of receiving surplus electrons by the n-typesemiconductor film 22 is inferior to that by not only the p-typesemiconductor film but also the metal base 2. The effect of thephotocatalyst film is reduced greatly when the n-type semiconductor film22 underlies the catalyst film.

FIG. 16 is a typical view of assistance in explaining, when a member hasa metal base 2 and an n-type semiconductor film 22, which is not aphotocatalyst film, a process of forming a p-type semiconductor film 21on the metal base 2. When the n-type semiconductor film 22 is removed bychemical or electrolytic decontamination, the p-type semiconductor film21 grows on the metal base 2 by a chemical reaction. When the n-typesemiconductor film 22 is removed by laser decontamination, only Cr canbe selectively left unremoved by using laser light 24 of a properwavelength. Since a chromium oxide forms a p-type semiconductor, onlythe p-type semiconductor film 21 remains on the surface of the metalbase 2. An n-type semiconductor film, i.e., a catalyst film, is formedon the p-type semiconductor film 21. The n-type semiconductor filmoverlying the p-type semiconductor film 21 exercises a high corrosionpotential reducing effect.

FIG. 17 is a graph showing the relation between the amount of a noblemetal attached to the surface of a structural member of a nuclearreactor, corrosion potential and the iron concentration of reactorwater. As obvious from FIG. 17, when the reactor water has a high ironconcentration, the effect of the noble metal is low and corrosionpotential increases with time. Such a variation of corrosion potentialoccurs also when a TiO₂ film is used. It is known from data shown inFIG. 17 that the reduction of the iron concentration of the reactorwater is effective in reducing corrosion potential by the presentinvention.

FIG. 18 shows a nuclear plant with a feedwater purifier by way ofexample, in which parts like or corresponding to those shown in FIG. 4are denoted by the same reference characters and the description thereofwill be omitted. The iron concentration of the feedwater must be limitedto a very small value to suppress the loose deposition of hematite onthe surface of the fuel. To reduce the iron concentration of the feedwater to a very small value, a purifier 25 needs to have a prefilterdevice 26 and a demineralizer device 27 in a serial arrangement.

FIG. 19 is diagrammatic view of a hydrogen/alcohol injecting system. Thehydrogen concentration of the reactor water can be effectively increasedby injecting hydrogen through a feedwater system injection lineconnected to a low-pressure part of a feedwater line into the nuclearreactor. The same effect as that of increased hydrogen concentration canbe achieved by injecting a liquid having alcohol groups, such asmethanol, into the nuclear reactor. Hydrogen or the liquid can beinjected into the nuclear reactor through a feedwater system injectionline 10 or a recirculation system injection line 9 connected to arecirculation line 8.

FIG. 20 is a diagrammatic view of a BWR nuclear plant including apressure vessel 5, a feedwater line 6, a purifying system 7, arecirculation line 8 and a corrosion potential measuring apparatus 11. Anoble metal composition injecting line 9 is connected to therecirculation line 8 to inject a noble metal composition into thereactor water. The reactor water is supplied to the corrosion potentialmeasuring apparatus 11. The corrosion potential measuring apparatus 11measures corrosion potential in the high-temperature reactor water. Whenapplying a noble metal to a reactor structural member, the noble metalcomposition injection lines 9 is actuated to inject the noble metalcomposition into the reactor water. The noble metal composition may beinjected into the reactor water while the nuclear reactor is in normaloperation, while the nuclear reactor is stopped or while the nuclearreactor is in a start-up stage, provided that an objective part of thereactor structural member to be subjected to corrosion potentialreduction is exposed to the reactor water. The relation betweeninjection time, temperature and concentration may be determinedbeforehand through experiments and the relation may be used forcontrolling a noble metal composition injecting operation or the noblemetal composition injecting operation may be controlled by monitoringthe process of the noble metal composition injecting operation by thecorrosion potential measuring apparatus 11 and the noble metalcomposition may be injected into the reactor water confirming thereduction of corrosion potential.

Although the noble metal composition. is injected into the reactor waterthrough the recirculation system in this embodiment, the noble metalcomposition may be injected into the reactor water through any partconnected to the nuclear reactor, such as a feedwater system, a residualheat removing system, a nuclear reactor cleaning system, a sampling lineor the like. The noble metal composition may be introduced into thenuclear reactor by a method that holds a sintered noble metalcomposition compact that discharges a noble metal composition in thenuclear reactor. The BWR nuclear plant has a purifying system forreducing the iron concentration of the feedwater to a sufficiently lowlevel. A small amount of hydrogen is injected into the nuclear reactorto make the noble metal exercise its catalytic effect.

As is apparent from the foregoing description, according to the presentinvention, the corrosion potential of the reactor structural member canbe reduced without injecting hydrogen into the reactor water or byinjecting a small amount of hydrogen into the reactor water to extendthe life of the reactor structural member.

It is possible to suppress the increase of the dose rate of the turbinesystem due to migration of volatile ammonia produced by the reaction ofhydrogen injected into the nuclear reactor with N-16 produced by nuclearreaction. Therefore, various facilities to reduce excess hydrogen in theoff-gas system by recombination can be reduced.

Furthermore, since only a very small amount of the noble metal isnecessary, the oxidation and hydrogenation of the fuel material are notpromoted substantially.

Although the invention has been described in its preferred embodimentswith a certain degree of particularity, obviously many changes andvariations are possible therein. It is therefore to be understood thatthe present invention may be practiced otherwise than as specificallydescribed herein without departing from the scope and spirit thereof.

1. A reactor structural member comprising: a surface adapted to belocated in a reactor water of a nuclear reactor; and a corrosionpotential reducing substance provided on the surface, the corrosionpotential reducing substance being formed as particles having a surfaceon which at least one of Pt, Rh, Ru and Pd is partially attached andbeing selected from the group consisting of a photocatalytic substancewhich produces an electromotive force under an irradiation of a light ora radioactive ray in the nuclear reactor and a metal or a metal compoundwhich forms the photocatalytic substance under a condition specified bya temperature and a pressure in the nuclear reactor, wherein thecorrosion potential reducing substance is a compound.
 2. The reactorstructural member according to claim 1, wherein the light in the nuclearreactor is a Cherenkov ray produced in a water-cooled nuclear reactor.3. The reactor structural member according to claim 1, wherein thephotocatalytic substance has a property of an n-type semiconductor. 4.The reactor structural member according to claim 1, wherein thecorrosion potential reducing substance at least one of adheres to or ispart of a film on the surface of the reactor structural member.
 5. Thereactor structural member according to claim 1, wherein a mass or athickness of the corrosion potential reducing substance is such that acurrent produced by the photocatalytic substance under the irradiationof the light or the radioactive ray is not lower than a sum of thresholdcurrent densities of an oxygen and a hydrogen peroxide contained in thereactor water.
 6. The reactor structural member according to claim 1,wherein the photocatalytic substance is one or more compound selectedfrom the group consisting of TiO₂, ZrO₂, PbO, BaTiO₃, Bi₂O₃, ZnO, WO₃,SrTiO₃, Fe₂O₃, FeTiO₃, KTaO₃, MnTiO₃ and SnO₂.
 7. The reactor structuralmember according to claim 1, wherein the corrosion potential reducingsubstance is an oxide of Ti or Zr, metal Ti, metal Zr, or a hydrate ofTi or Zr.
 8. The reactor structural member according to claim 1, furthercomprising a corrosion oxide film formed on the surface of the reactorstructural member; wherein an adhesiveness of the corrosion potentialreducing substance to the corrosion oxide film formed on the surface ofthe reactor structural member is enhanced by providing a hydrophilicproperty or by mixing a binder substance.